U.S. patent number 6,645,361 [Application Number 09/936,815] was granted by the patent office on 2003-11-11 for electrochemical gas sensor.
This patent grant is currently assigned to Robert Bosch GmbH. Invention is credited to Bernhard Bloemer, Detlef Heimann, Thomas Moser, Harald Neumann, Hans-Joerg Renz, Margret Schuele, Bernd Schumann, Carsten Springhorn, Rainer Strohmaier, Sabine Thiemann-Handler.
United States Patent |
6,645,361 |
Bloemer , et al. |
November 11, 2003 |
**Please see images for:
( Certificate of Correction ) ** |
Electrochemical gas sensor
Abstract
An electrochemical gas sensor for determining the concentration
of gas components in a gas mixture, in particular for determining
NOx and HC. The gas sensor includes a first measuring gas space
which is connected to the measuring gas, and a second measuring gas
space which is connected to the first measuring gas space by a
connecting channel. Furthermore, a first electrode and a second
electrode, arranged in the first measuring gas space, and at least
one third electrode arranged in the second measuring gas space, and
at least one fourth electrode are provided. The two measuring gas
spaces are arranged in layer planes on top of one another and are
separated from one another by at least one oxygen ion conducting
layer, the connecting channel passing through the oxygen ion
conducting layer.
Inventors: |
Bloemer; Bernhard (Stuttgart,
DE), Strohmaier; Rainer (Stuttgart, DE),
Springhorn; Carsten (Stuttgart, DE), Heimann;
Detlef (Gerlingen, DE), Renz; Hans-Joerg
(Leinfelden-Echterdingen, DE), Neumann; Harald
(Farmington Hills, MI), Schuele; Margret (Weil der Stadt,
DE), Schumann; Bernd (Rutesheim, DE),
Moser; Thomas (Schwieberdingen, DE),
Thiemann-Handler; Sabine (Stuttgart, DE) |
Assignee: |
Robert Bosch GmbH (Stuttgart,
DE)
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Family
ID: |
7901433 |
Appl.
No.: |
09/936,815 |
Filed: |
January 14, 2002 |
PCT
Filed: |
March 11, 2000 |
PCT No.: |
PCT/DE00/00769 |
PCT
Pub. No.: |
WO00/57169 |
PCT
Pub. Date: |
September 28, 2000 |
Foreign Application Priority Data
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Mar 18, 1999 [DE] |
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199 12 102 |
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Current U.S.
Class: |
204/426; 204/425;
204/427; 205/781 |
Current CPC
Class: |
G01N
27/417 (20130101) |
Current International
Class: |
G01N
27/417 (20060101); G01N 027/407 () |
Field of
Search: |
;204/421-429 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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198 27 469 |
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Jan 1999 |
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DE |
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0 678 740 |
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Oct 1995 |
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EP |
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0 869 356 |
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Oct 1998 |
|
EP |
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0 897 112 |
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Feb 1999 |
|
EP |
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Primary Examiner: Tung; T.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. An electrochemical gas sensor for determining concentration of
gas components in a gas mixture comprising: a first measuring gas
space in contact with a measuring gas, the first measuring gas
space including a first electrode and a second electrode; a second
measuring gas space including at least one third electrode, the at
least one third electrode including two partial electrodes arranged
opposite one another in the second measuring gas space; a
connecting channel connecting the first measuring gas space to the
second measuring gas space; at least one oxygen ion conducting
layer separating the first measuring gas space from the second
measuring gas space; and at least one fourth electrode; wherein the
first and second measuring gas spaces are arranged in layer planes
on top of one another.
2. The gas sensor of claim 1, further comprising: at least two gas
inlet openings leading into the first measuring gas space, the at
least two gas inlet openings being arranged symmetrically with
respect to a plane passing through a center line of the connecting
channel.
3. The gas sensor of claim 1, wherein the at least one third
electrode includes a material capable of reducing a No.sub.x gas
component.
4. The gas sensor of claim 3, wherein the at least one third
electrode includes one of rhodium and a rhodium/platinum alloy.
5. The gas sensor of claim 1, wherein the first and second
electrodes are arranged in series in the first measuring gas space
in a direction of diffusion of the measuring gas.
6. The gas sensor of claim 1, wherein the first electrode includes
a material that does not fully catalyze an establishment of
equilibrium of the gas mixture.
7. The gas sensor of claim 1, wherein the first electrode includes
one of gold and a platinum/gold alloy.
8. The gas sensor of claim 1, further comprising: an additional
electrode exposed to the measuring gas, a pump voltage being
applied between the additional electrode and the first electrode to
establish a constant oxygen partial pressure in the first measuring
gas space; wherein the at least one third electrode and the at
least one fourth electrode form an additional pump cell, the pump
cell generating a pump current picked up as a measuring signal.
9. The gas sensor of claim 1, wherein the second electrode and the
at least one fourth electrode form a concentration cell, the
concentration cell being controlled at a constant voltage.
10. The gas sensor of claim 1, wherein the at least one third
electrode and the at least one fourth electrode are coupled and
form pump cells for pumping oxygen into the second measuring gas
space, the first electrode being designed as a mixed potential
electrode and forming a concentration cell with the at least one
fourth electrode, the voltage of the concentration cell being
picked up as a measuring signal.
11. The gas sensor of claim 1, wherein: the two partial electrodes
are not in direct physical contact with each other.
12. The gas sensor of claim 1, wherein: the connecting channel
includes a bore.
13. The gas sensor of claim 1, wherein: the two partial electrodes
include a first partial electrode and a second partial electrode,
the first partial electrode performs a first function, and the
second partial electrode performs a second function.
14. An electrochemical gas sensor for determining concentration of
gas components in a gas mixture, comprising: a first measuring gas
space in contact with a measuring gas, the first measuring gas
space including a first electrode and a second electrode, the first
electrode including a material that does not fully catalyze an
establishment of equilibrium in the gas mixture; a second measuring
gas space including at least one third electrode, the at least one
third electrode including a material capable of reducing NO.sub.x
gas components, the at least one third electrode including two
partial electrodes arranged opposite one another in the second
measuring gas space; a connecting channel connecting the first
measuring gas space to the second measuring gas space; and at least
one fourth electrode.
15. The gas sensor of claim 14, wherein the first electrode
includes one of gold and a platinum/gold alloy.
16. The gas sensor of claim 15, wherein the platinum/gold alloy
includes 0.5 to 3 wt. % of gold.
17. The gas sensor of claim 16, wherein the platinum/gold alloy
includes approximately 1 wt. % of gold.
18. The gas sensor of claim 14, wherein the at least one third
electrode includes one of rhodium and a rhodium/platinum alloy.
19. The gas sensor of claim 14, further comprising: at least one
oxygen ion conducting layer separating the first measuring gas
space from the second measuring gas space; wherein the first and
second measuring gas spaces are arranged in layer planes on top of
one another and the connecting channel passes through the at least
one oxygen ion conducting layer.
20. The gas sensor of claim 14, wherein: the two partial electrodes
are not in direct physical contact with each other.
21. The gas sensor of claim 14, wherein: the connecting channel
includes a bore.
22. The gas sensor of claim 14, wherein: the two partial electrodes
include a first partial electrode and a second partial electrode,
the first partial electrode performs a first function, and the
second partial electrode performs a second function.
Description
The present invention relates to an electrochemical gas sensor for
determining the concentration of gas components in gas mixtures, in
particular for determining the concentration of NOx and HC
according to the preamble of independent claims 1 and 13.
BACKGROUND INFORMATION
European Patent Application 678 740 A1 describes a gas sensor of
the aforementioned type for determining the NOx concentration in a
gas mixture, in which two measuring gas spaces, each containing a
pump cell, are arranged next to one another in a layer plane of a
planar, oxygen ion conducting ceramic substrate. The measuring gas
flows through a first diffusion opening into the first measuring
gas space in which a first internal pump electrode is arranged. An
external pump electrode is directly exposed to the measuring gas.
The first internal pump electrode and the external pump electrode
form the first pump cell. A predetermined oxygen partial pressure
is set in the first measuring gas space via the first pump cell by
pumping oxygen in and out. A concentration cell (Nernst cell) has a
measuring electrode and a reference electrode connected to the
atmosphere; the measuring electrode is arranged in the first
measuring gas space. In order to set a constant oxygen partial
pressure in the first measuring gas space, the voltage
(electromotive force) of the concentration cell is controlled at a
constant value via a pump voltage of the first pump cell. The first
and second measuring gas spaces are connected by a connecting
channel, which represents another diffusion opening; the atmosphere
set at a constant oxygen partial pressure diffuses into the second
measuring gas space via the connecting channel. Another internal
pump electrode, which works together with the reference electrode
arranged in the atmosphere reference channel and forms the second
pump cell, is arranged in the second measuring gas space. The
second internal pump electrode is made of a material, for example,
rhodium, which reduces NO to N.sub.2 and O.sub.2. The reduced
oxygen occurring at the second internal pump electrode is pumped,
via a voltage applied to the pump, to the reference electrode and
there it is released into the atmosphere. Since the atmosphere in
the first measuring gas space is held at a constant oxygen partial
pressure, the pump current for removing the reduced oxygen from the
second measuring gas space is proportional to the NOx
concentration. The measuring gas spaces and pump cells arranged in
series are kept at different temperatures, the temperature at the
electrodes in the first measuring gas space being set lower than
the temperature at the electrode in the second measuring gas space.
The sensor element design is relatively complicated and is only
suitable for determining NOx.
ADVANTAGES OF THE INVENTION
The gas sensor according to the present invention having the
characterizing features of claim 1 has the advantage that a basic
sensor routinely manufactured for determining the lambda value of
gas mixtures is used, to which only at least one further solid
electrolyte layer having two additional electrodes has to be added.
A sensor known as broad-band sensor, having a pump cell containing
an internal and external pump electrode and a concentration cell
containing a measuring electrode and a reference electrode, is used
as the basic sensor. The measuring electrode and the internal pump
electrode are arranged in a gas space opposite one another. In the
sensor element according to the present invention this gas space
forms the second measuring gas space. The use of a routinely
manufactured basic lambda sensor offers considerable cost
advantages compared to a sensor element design that is specialized
for each application.
Another aspect of the present invention is, according to the
characterizing features of claim 8, that the gas sensor can be used
both as an NOx sensor and as an HC sensor with a single sensor
element design. Only the taps at the electrode terminals are to be
selected and the analysis circuit is to be adapted accordingly for
either application. When the gas sensor is used as an HC sensor,
the first electrode arranged in the first measuring gas space is
used as a mixed potential electrode. When the gas sensor is used as
an NOx sensor, the third electrode arranged in the second measuring
gas space is used as an NOx reducing electrode. Additional cost
advantages result through the wide application of the gas
sensor.
The measures described in the subclaims allow advantageous
refinements of and improvements on the sensor element presented in
the main claim. The arrangement of two gas inlet openings symmetric
with respect to the connecting channel ensures sufficient gas
exchange of the measuring gas with the first measuring chamber and
thus a short response time. In addition, this allows higher pump
currents for the first pump cell.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the present invention is illustrated in the
drawing and described in detail in the description that
follows.
FIG. 1 shows a cross-section through a sensor element of a gas
sensor according to the present invention.
FIG. 2 shows a section through a layer plane according to line
II--II in FIG. 1, and
FIG. 3 shows a section through a layer plane according to line
III--III in FIG. 1.
DETAILED DESCRIPTION OF THE EMBODIMENT
FIGS. 1, 2, and 3 show a schematic design of a preferred embodiment
of the present invention. A planar sensor element 10 of an
electrochemical gas sensor has, for example, a plurality of oxygen
ion conducting solid electrolyte layers 11a, 11b, 11c, 11d, 11e,
11f, 11g, and 11h. Solid electrolyte layers 11a to 11h are designed
as ceramic foils and form a planar ceramic body. The integrated
form of the planar ceramic body of sensor element 10 is produced by
laminating together the ceramic foils imprinted with function
layers and subsequent sintering of the laminated structure in a way
that is known per se. Each solid electrolyte layer 11a to 11h is
made of oxygen ion conducting solid electrode material such as
stabilized ZrO.sub.2, for example.
Sensor element 10 has a first measuring gas space 13 and a second
measuring gas space 15. The two measuring gas spaces 13, 15 are
located on top of one another in different layer planes and are
connected to one another by a connecting channel 17, designed as a
bore hole, for example. Independently of the two measuring gas
spaces 13, 15, a reference gas channel 19, whose one end leads out
of the planar body of sensor element 10 and is connected to the
atmosphere, is arranged, for example, in an additional layer
plane.
Sensor element 10 also has two gas inlet openings 21, which conduct
the measuring gas into first measuring gas space 13. The two gas
inlet openings 21 are arranged symmetrically with respect to a
plane passing through the center line of connecting channel 17, for
example (FIG. 3). A first diffusion barrier 23 made of porous
ceramic material, for example, is formed at the inlets to first
measuring gas space 13 in the direction of diffusion of the
measuring gas downstream from gas inlet openings 21. It is also
conceivable that gas inlet openings 21 themselves be filled with a
porous ceramic material, so that diffusion barrier 23 is then
located within gas inlet openings 21.
A second diffusion barrier 25 is formed in the direction of
diffusion of the measuring gas at the end of connecting channel 17
and upstream from second measuring gas space 15. Second measuring
gas space 15 has a circular design, for example, so that second
diffusion barrier 25 is also positioned annularly around the
opening, located at the flow direction end of connecting channel
17.
A first internal electrode 27 and a second internal electrode 31
are arranged in first measuring gas space 13. An external electrode
28, which may be covered by a porous protective layer (not shown)
and is directly exposed to the measuring gas, is located on the
external large surface of solid electrolyte layer 11a. In the
present embodiment, internal electrodes 27, 31 are arranged in
series in the direction of diffusion of the measuring gas. It is,
however, also possible to arrange internal electrodes 27, 31
opposite one another in the first measuring gas space.
A third internal electrode 35, which has two annular partial
electrodes opposite one another in the present embodiment, is
located in second measuring gas space 15. An additional electrode
33, exposed to atmospheric air, is located in reference channel 19.
However, an embodiment in which fourth electrode 33 is also exposed
to the measuring gas is also conceivable.
In order to use sensor element 10 both as an NOx sensor and an HC
sensor, first internal electrode 27 and third internal electrode 35
must be made of special electrode materials.
For the mode of operation as an HC sensor, first internal electrode
27 is made of a material that lets this electrode operate as a
mixed potential electrode, the mixed potential electrode not being
capable or not being fully capable of catalyzing the establishment
of gas equilibrium of the gas mixture. Gold or a gold/platinum
alloy preferably having 1 wt. % gold is such a material, for
example. External electrode 28, second internal electrode 31, and
additional electrode 33 are made of a catalytically active
material, for example, platinum.
For the mode of operation as an NOx sensor, third internal
electrode 35 arranged in second measuring gas space 15 is made of a
material capable of catalytically reducing NOx. Rhodium or a
rhodium/platinum alloy is such a material, for example. It is
important for operation as an NOx sensor that electrodes 27, 31,
upstream from NOx-sensitive electrode 35 in the direction of
diffusion of the measuring gas allow essentially no reduction of
NOx. The electrode material for all electrodes is used in the known
manner as cermet to be sintered with the ceramic foils.
According to FIGS. 2 and 3, contact points 42 of electrodes 27 and
31, formed on the surface of sensor element 10, are connected to
printed conductors 41 running in the layer plane between solid
electrolyte layers 11b and 11c. Contact point 44 of external
electrode 28, leading to the layer plane underneath it and forming
another contact point 45 there, is connected to a printed conductor
43 on the large surface of solid electrolyte layer 11a. The two
partial electrodes opposite one another of electrode 35 are
contacted within the ceramic substrate and with additional printed
conductors (not shown) run to additional contact points 46 formed
on the surface of sensor element 10 like additional electrode
33.
An electric resistance heater 39 is furthermore embedded in the
ceramic body of sensor element 10 between two electrical insulation
layers not shown in detail. Resistance heater 39 is used for
heating sensor element 10 to the required operating temperature.
Electrodes 27, 28, 31, 33, 35 arranged essentially on top of one
another are exposed to essentially the same temperature. No attempt
is made to set specific temperature differences at the individual
electrodes, which would not be possible. Resistance heater 39 has
heater contact points (not shown) on the external large surface of
sensor element 10 opposite contact points 42, 45, 46.
The structure of sensor element 10 according to the present
invention as shown in FIG. 1 uses a broad-band sensor for
determining the lambda value as a basic sensor. The basic sensor is
formed by solid electrolyte layers 11c, 11d, 11e, 11f, 11g, and 11h
and by electrodes 27, 33, and 35. Electrodes 27 and the first
partial electrode of electrode 35, opposite electrodes 27 in
measuring gas space 15 form a pump cell in the broad-band sensor,
and the second partial electrode of electrode 35, together with
additional electrode 33, forms the concentration cell, electrode 33
acting as a reference electrode. As a refinement of sensor element
10 according to the present invention, electrodes 27 and 28 of
solid electrode layers 11a and 11b are connected to the basic
sensor, measuring gas space 13 being located in solid electrolyte
foil 11b. However, an embodiment in which only solid electrolyte
foil 11a is used is also conceivable. In this embodiment, measuring
gas space 13 is then also integrated in solid electrolyte foil
11a.
The above-described sensor element 10 can be used both as an NOx
sensor and as an HC sensor, individual electrodes 27, 28, 31, 33,
35 performing different functions depending on the application. For
this purpose, electrodes 27, 28, 31, 33, 35 are electrically
interconnected according to the functions of the electrodes.
Operation as an NOx Sensor
When sensor element 10 is used as an NOx sensor, external electrode
28 and first internal electrode 27 are operated as pump electrodes
of a first pump cell. Second internal electrode 31 is wired with
additional electrode 33 acting as a reference electrode as a
concentration cell. A pump voltage is applied to electrodes 27, 28,
through which a constant oxygen partial pressure is set in first
measuring gas space 13 by pumping oxygen in or out. The pump
voltage applied to electrodes 27, 28 is controlled so that a
constant voltage value, for example, 450 mV, is set at electrodes
31, 33 of the concentration cell. This voltage corresponds to a
lambda value=1. For a lean measuring gas (lambda>1), oxygen is
pumped by the first pump cell out of first measuring gas space 13.
For a rich measuring gas (lambda<1), oxygen is pumped into first
measuring gas space 13 from the measuring gas. By selecting the
electrode material and/or by applying an appropriate pump voltage,
it is guaranteed that no NOx is pumped away at electrodes 27, 31
when pumping oxygen.
The measuring atmosphere adjusted to a constant oxygen partial
pressure is now pumped via connecting channel 17 and second
diffusion barrier 25 to second measuring gas space 15. Third
internal electrode 35 located in second measuring gas space 15 is
operated, together with additional electrode 33, as a second pump
cell. Because of the catalytic material, third internal electrode
35 acts as an NOx-sensitive electrode, at which the NOx is reduced
according to the reaction NO.fwdarw.1/2N.sub.2 +1/2O.sub.2.
Reference electrode 33, working together with electrode 31 operates
simultaneously as a second pump electrode, at which the oxygen
pumped away from second measuring gas space 15 is released to the
atmosphere. Due to diffusion barrier 25 forming a diffusion
resistance, the NOx diffusing into second measuring gas space 15 is
fully pumped away from electrode 35. Thus a limit current is
established at the electrochemical cell acting as an additional
pump cell, providing the NOx concentration when picked up as a
measuring signal.
Operation as an HC Sensor
If sensor element 10 is used as an HC sensor, electrodes 27, 28,
31, 33, 35 are interconnected in a manner that is different from
that used for the NOx sensor application. Electrodes 33, 35 are
operated as pump electrodes but, contrary to the NOx application,
so that oxygen is pumped from the atmosphere into second measuring
gas space 15. In doing so, an artificial measuring atmosphere with
a higher oxygen concentration (lambda>1) compared to the
measuring gas is created in second measuring gas space 15, the
measuring atmosphere being pumped back into first measuring gas
space 13 via the connecting channel. First diffusion barrier 23
prevents oxygen from escaping unhindered into the measuring gas, a
higher oxygen partial pressure being maintained in first measuring
gas space 13.
It is important for the mode of operation as an HC sensor that
first internal electrode 27 be a "poisoned" catalytically active
electrode, which does not establish or at least does not fully
establish equilibrium of the gas mixture as a mixed potential
electrode. Operation as an HC sensor also makes use of the fact
that a measuring signal at the electrochemical sensors,
representing the HC concentration, differs from the oxygen
concentration signal curve in a characteristic manner in lean
measuring gas (lambda>1) only.
Due to the fact that the higher oxygen concentration in first
measuring gas space 13 differs from that in reference channel 19 a
first voltage is established between electrode 27 acting as a mixed
potential electrode and electrode 33, which correlates with the HC
concentration and the oxygen concentration difference. A second
voltage signal between catalytically active electrode 31 and
electrode 33 corresponds to the oxygen concentration in measuring
gas space 13. The difference between the two voltages corresponds
to the HC concentration in the measuring gas. This voltage
difference is also applied between electrode 27 and electrode 31,
so that the voltage picked up between electrodes 27, 31 provides
the HC concentration as a measuring signal. It is, however, also
possible to pick up the current driven by the voltage between
electrodes 27, 31 as the measuring signal.
Electrode 28 has no function in the case of an HC sensor and is
therefore not taken into consideration in the wiring of sensor
element 10.
In another embodiment of the present invention a special sensor
element is used for each application as an NOx sensor and an HC
sensor. In this case, preferably the same layer structure of the
solid electrolyte layers is used for both the NOx sensor and the HC
sensor. There are differences in the material for electrode 27 and
electrode 35. In an NOx sensor, electrode 31 cannot be manufactured
from a mixed potential material, but may be made of the same
material as electrode 27. In the case of an HC sensor, the material
of electrode 35 may not be NOx reducing, but can be made of the
same material as electrode 31 or even electrode 27. Since the
electrodes are applied to the ceramic foils using thick layer
technology, such an embodiment still offers cost advantages, since
the number of foils and the expensive lamination method remain
identical.
In addition, the sensor is suitable for determining ammonia in gas
mixtures. For this purpose, platinum is embedded in first diffusion
barrier 23 which oxidizes ammonia to NOx due to its catalytic
effect.
* * * * *